A disk drive stores data to a magnetic disk and retrieves data from the magnetic disk. The data is encoded in a digital format to allow efficient manipulation of the data by devices that process data read from the disk drive. The disk drive does not directly store digital data. Instead, the disk drive stores analog signals that represent the digital data. Disk drives encode digital information as analog magnetic flux changes on spinning magnetic disks. The magnetic flux changes are read by a sensor in the disk drive and converted to an analog signal. The sensor is moved on a mechanical arm to a position from where the flux changes are read. The analog signal is decoded to recover the digital data. A circuit that reads the encoded data and reproduces the original digital data is referred to as a read channel.
Even though disk drives have been widely used for many years, a memory system using a disk drive has drawbacks. The disk drive executes operations in a serial manner because the mechanical arm with the sensor needs to be moved from one position to the next when data is accessed on different disk drive tracks. The disk drive also takes time to speed up from an idle state before memory can be accessed. A wobbly magnetic disk may not be reliably read. Temperature may also affect the reliability of the magnetic disk. In an ideal theoretical environment, each magnetic flux change of a magnetic disk represents a bit value of zero or one. In practice, the physical proximity of one recorded bit to the next adjacent bit tends to cause interference between the adjacent bits. This interference eventually leads to digital bit errors when the analog signal is converted to a digital signal. For these reasons a more reliable memory system may be desired.